The present invention relates to a scanning probe microscope, and more specifically, relates to a scanning probe microscope which scans the surface of a sample and makes a measurement while oscillating a cantilever to which an exploratory needle is attached near its resonance point.
In order to observe the surface or make a measurement of the surface roughness of metal, semiconductor material, ceramic and synthetic resin, a scanning probe microscope (SPM) as the representative of an atomic force microscope (AFM) which makes the measurement of an atomic force generated between the exploratory needle (probe) and the surface of the sample, is widely known. In the atomic force microscope, several measurement modes are used. However, recently, means called a non-contact mode or dynamic mode which makes the cantilever to which the exploratory needle is attached oscillate near the resonance point, converts and detects an interaction between the cantilever in the oscillated state and the surface of the sample applying to the exploratory needle into change in oscillation amplitude, phase, or frequency of the cantilever, is largely used (for example, refer to Japanese Patent Publication (TOKKAI) No. H11-2637 (paragraph 0003-0004).
FIG. 5 is a principled configuration diagram of the detecting portion of the atomic force microscope which is conventionally well-known. At the distal end of a cantilever 10, a sharp exploratory needle 11 is attached, and a basal portion 12 of the cantilever 10 is fixed to a pedestal 14 wherein a ceramic piezoelectric element 13 is mounted. The piezoelectric element 13 or pedestal 14 is held by a cantilever holder (not shown), and the cantilever holder is fixed to the device (microscope). The piezoelectric element 13 changes its shape in its thickness direction (up-and-down direction in FIG. 5) by the voltage which will be applied, so that when the alternating voltage with a predetermined frequency f is applied from a drive portion 15, the cantilever 10 moves up and down by the oscillation of the piezoelectric element 13. In the above-mentioned non-contact mode or dynamic mode, the frequency f of the alternating voltage is set near a mechanical resonance point of the cantilever 10. Herewith, in the cantilever 10, oscillation with amplitude larger than the oscillation amplitude of the piezoelectric element 13 itself is induced. Incidentally, displacement of the cantilever 10 is detected by an optical detection mechanism (not shown).
At this point, the displacement magnitude of the piezoelectric element 13 depends on the material or shape of the piezoelectric element 13 besides the amplitude of the alternating voltage which will be applied. Also, the oscillation amplitude of the cantilever 10 changes according to the spring constant which depends on the material or shape of the cantilever 10, condition of contact between the basal portion 12 of the cantilever 10 and the pedestal 14 besides the displacement magnitude of the piezoelectric element 13. Even if it is the same type of cantilever 10, since the individual difference of the spring constant is measurable, when the alternating voltage with the same amplitude is applied, a relatively large difference in the oscillation amplitude of the cantilever 10 may be observed.
In the atomic force microscope, the amplitude of the cantilever 10 is one of the important measurement parameters, and according to the material or shape of a sample S to be observed, an operator is required to adjust the amplitude of applied voltage so as to gain an appropriate amplitude. However, even if the voltage applied to the piezoelectric element 13 is the same, if the type or shape of the cantilever 10 differs as mentioned above, the oscillation amplitude also differs. As a result, in the case wherein the cantilever 10 which is difficult to be oscillated is used, even if the maximum voltage is applied to the piezoelectric element 13, the amplitude suitable for the most appropriate observation condition may not be achieved. On the other hand, when the cantilever 10 which is easily oscillated is used, even if the minimum voltage which can be stably-generated at the drive portion 15 is applied to the piezoelectric element 13, the amplitude of the cantilever 10 may become larger than the amplitude suitable for the most appropriate observation condition. More specifically, in the conventional scanning probe microscope, since the dynamic range of the adjustment of the oscillation amplitude of the cantilever 10 is narrow, there may be a case wherein the measurement cannot necessarily be performed on the most appropriate condition. Accordingly, this may contribute to, for example, deterioration of accuracy of an observation image.
This invention is made in order to solve the above-mentioned problems, and the main purpose of the invention is to provide a scanning probe microscope which can enlarge the dynamic range of the adjustment of oscillation amplitude of the cantilever when the cantilever is oscillated at a frequency near its resonance point.
Further objects and advantages of the invention will be apparent from the following description of the invention.